Question

Find the sum of terms from 21st term to50th term, of AP 1, 5, 9, 13,........

Answer 1

Answer:

4170

Explanation:

Here , First term , a = 1 ,

Common Difference, d= 5-1= 4

Now, 20th term = a + 19d

= 1+19×4

= 77

50th term = a+49d

= 1+ 49×4

= 197

Sum of 21st term to 50th term=

Sum upto 50th term - Sum upto 20th term

= 50/2( 1+ 197) - 20/2( 1+77)

= 25× 198- 10× 78

= 4950- 780

= 4170

[Formula used:

Sum of n terms of AP = n/2 ×(first term+ last term)]

Answer 2

Explanation:

What is the 21st term of an A.P. whose first two terms are -3 and 4.